Just to pitch in that it seems in vogue for running magazines and other advice columns to say that no one should even think about going over two hours of running without recource to energy gels, or hypotonic drinks. Perhaps this is because there is a growing industry in making these products and selling them to unconfident and unknowing runners and cyclists. It wouldn't be the first time that money has got in the way of objective advice giving (e.g. sports footwear industry anyone?).
I looked around for information about hydration and energy supplements before trying my first marathon and everywhere took it as a given that participants at that distance would be having something or other. I would just say:
Go with what your body is telling you and not what other people say your body should be getting.
I have never used drink or food on any run I have ever done, whatever the distance or time. This includes the marathon I got around. So, it works for me and keeps things simple. Mind you, living in the UK, the weather doesn't tend to get too hot so fluid loss during running is not such a problem. though it sounds obvious, when the weather is hot, I keep sessions shorter or run at times when it's cooler.
Like others have said before though, everyone is different and their goals may differ too. Here are a few things I have heard about:
1. Getting dehydrated even a little bit can have a more significant effect on performance. If you're into PBs, ultra distances, etc, then keeping hydrated is in one's best interests - mind you, I've heard lots of stories of hyperhydration (drinking too much for immediate requirements) so it's best to use discretion.
2. Studies have shown that training without eating/drinking before/during training improves fitness and performance over time more than those who don't train "on an empty stomach".
3. Many sources recommend only taking drink/fluids if running extends beyond two hours. Also, it should be in the form of carbohydrates to help keep glycogen stores up. The body can be trained to rely on burning fat for energy rather than carbohydrates but carbs are still needed in burning fat energy so it needs replenishing over an extended period of time.
4. I have found no problems with a lack of intake during the course of any run. Again, there appears to be a consensus that it is important to eat and drink within a 15-20 minute timeframe after the discontinuation of exercise to speed recovery and repair. This is in part due to an increased metabolic uptake for a time after exercise.
I suppose the point I most want to make is have the confidence to do what feels right for you and not be swayed too much by the current trends. I have the confidence now to know that I can run 26 miles and 4 hours+ without food or water. I wouldn't have discovered this if i'd followed the advice of the running magazines and "experts". Good luck with whatever you choose.
